Return null in HtmlAgilityPack

I want use this code but it returns NULL in all this parameters ! would you please help how to fix it or what is the problem ?

click here for a picture that shown what happened.

code :

    HtmlWeb hw = new HtmlWeb();

    private void button1_Click(object sender, EventArgs e)
        {
            Crawler("http://www.avval.ir/directory", 0);
        }

    public void Crawler(string url, int pid)
        {


            try
            {

                HtmlAgilityPack.HtmlDocument doc = hw.Load("http://www.avval.ir/directory");
                HtmlNodeCollection nodes = doc.DocumentNode.SelectNodes("//a[@class='metatag-topi metatag-keywords']");
                HtmlNodeCollection nodes2 = doc.DocumentNode.SelectNodes("//div[@class='pagination']/a[@href]");
                HtmlNodeCollection phone = doc.DocumentNode.SelectNodes("//p[@class='phone fRight ml10']");

            }
        }